This invention relates to a dewatering arrangement at the forming of a web from fibrous particles suspended in water. The invention particularly refers to a device applicable to the wet end of a papermaking machine.
In SE-PS No. 7904555-5 a method of forming paper is described where the forming process proper takes place between a flexible upper lip and an unsupported portion of the wire over an open suction box. The flexible upper lip is an extension of the stationary upper lip on a head box. This implies that the stock jet from the head box is directed downward to the sheet forming zone by the flexible lip, and that during the entire sheet forming process the stock layer is clearly defined as to its form. The absence of disturbances in the surface layer of the stock jet is a prerequisite for the grammage variations in the paper made to be maintained on a lower level. An additional improvement of the uniformity of the paper is obtained when the forming takes place with the influence of viscous shearing forces between the stationary upper lip and the movable wire. Due to the effect of the pressure difference between the atmospheric pressure and the low pressure in the suction box, the wire moves in a curved path. The flexible upper lip adapts itself to the form of the wire and thereby also assumes curved form. For producing the vacuum in the suction box, the box must be sealed on the sides. This sealing is made by means of adjustable sealing strips. This implies that, irrespective of the configuration of these strips, the wire will be curved three dimensionally along the sides over the suction box. The flexible upper lip, which should have a certain bending resistance, cannot fully adapt itself to the three dimensional form of the wire. Hereby certain edge disturbances can arise in the paper sheet formed, which may cause problems for the operativeness of the papermaking machine. These problems can be neglected for low paper grammages and for stocks easy to dewater, because only small pressure differences over the wire are required, resulting in a small downward deflection of the wire. For higher grammages, however, and for stocks more difficult to dewater higher pressure differences and also longer dewatering distances are required, which together result in a substantial downward deflection of the wire and consequently also in increased edge disturbances in the sheet formed.
A further disadvantage of the method is that great amounts of energy are required for generating the vacuum in the suction box.
One way of avoiding the aforesaid disadvantages, but still utilizing the advantages, is to generate the necessary dewatering pressure by means of an overpressure above the flexible lip over an unsupported portion of the wire. In this case the upper lip and wire are loaded in their entire extension across the papermaking machine, whereby only a two dimensional deflection is caused. The overpressure can be produced by means of an air cushion or a rigid pressure plate, which is designed so as to face the stock with a convex surface. In the case of a convex pressure plate, the plate can be utilized together with a flexible upper lip or it can entirely replace the same. Such an arrangement is proposed in U.S. Pat. No. 4,416,730.
In the U.S. patent the pressure plate (in said U.S. Pat. No. 4,416,730 called slide shoe) is described generally to have a surface curved convex toward the stock. The pressure plate further is rigidly connected to the head box and can be regarded as an extension of the upper (or lower lip) of the head box. In embodiments of the patent, the stock jet is transported after the outlet opening along a convex surface where the opposed surface, at least for a certain distance, is a free liquid surface. The disadvantage of this method will be described in greater detail below where also the invention, on which this application is based, will be described.